Vehicle positioning methods and apparatus with impact damper

ABSTRACT

A push roll assembly mounted to a material receiving apparatus, such as a paver or other material receiving machine, includes a resilient link which allows energy transferred during contact of a supply truck with the push rolls to be gradually transferred to the material receiving machine, to thereby smoothen the impact between such truck and the machine.

This is a division of copending application Ser. No. 07/448,055 filed onDec. 8, 1989, now U.S. Pat. No. 5,004,394.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to vehicle positioning methods andapparatus, such as vehicle stops and particularly to positioning stopsknown as truck push rolls and methods of positioning trucks by use ofsuch truck push rolls. Such truck push rolls may be used in locating therear of a material dump truck with respect to a dump location andparticularly with respect to a continuously advancing dump location, forexample, a feed hopper of a roadway paving machine.

2. Discussion of the Prior Art

Paving apparatus for laying down continuous strips of pavementnecessarily requires a continuous supply of paving material. Typically,trucks deliver the material to the paving site. According to onepreferred manner of operation, supply trucks with unloading provisions,such as tiltable beds, back up against a feed or supply hopper locatedat a front end of a paving machine or paver. When the rear of the truckbed is properly positioned with respect to the front of the feed hopper,the bed is raised and the paving material, such as asphaltic aggregatematerial, is transferred to the feed hopper of the paver.

Construction equipment, such as dump trucks, paving machines or similartype of apparatus are typically constructed for rugged use. Thustypically the equipment does not include bumpers, or even shockabsorbing bumpers, as may have been provided for automobiles or evenboat hulls for docking purposes. The intent of such shock absorbingbumpers is to protect the respective vehicle from damage due to contactwith another object. The typical ruggedness with which the constructionapparatus of interest has been regarded as not needing shock absorbingbumpers to afford such equipment protection from damage. A prior artproblem of positioning trucks with respect to the feed hopper in theabsence of bumpers was solved to accommodate the needs of the supplyoperation performed while the paving machine is constantly advancing.

The truck typically remains in position with respect to the feed hopperof the paver only while the material is being dumped into the hopper andimmediately thereafter drives off to give way to the next truck. Thechange-over time between trucks is preferably minimized to optimize thetime allocated to actually dumping the material into the feed hopper.Thus to quickly position each of the trucks with respect to the feedhopper, a truck positioning device known as truck push rolls is mountedto the front of the feed hopper of the respective machine to which thematerial is being transferred. Such push rolls are horizontally disposedrolls which are spaced to coincide with the track width of the trucks.

There are two types of state of the art push roll assemblies, push rollswith so called "truck hooks" and those without. The function of truckhooks is well known in the art. Truck hooks are retainers of truckswhich typically engage both outer rear wheels of a truck in a clamshelltype manner after the truck has become positioned against the pushrolls. The hooks may be engaged manually, by overcentering mechanisms,or by hydraulic cylinders in a direct or remote operation. On levelroadways and even on slight descending slopes, roughness of base gradesmay provide sufficient frictional resistance to retain supply trucksagainst push rolls, such that the use of truck hooks becomesunnecessary. Also, during the brief stop of a supply truck against apositioning stop, such as a push roll assembly, a truck operator mayapply a slight pressure on the vehicle's brakes to maintain contact withthe push rolls. In the further discussion of the invention and itsbackground, it should be understood that problems addressed by thecurrent invention relate to positioning the trucks. The currentinvention consequently applies equally to push roll assemblies with andwithout the so-called truck hooks.

Thus, in both instances, trucks back their wheels against the rollswhich remain in contact with the truck wheels while the truck is dumpingits load of material. A support structure of the rolls is typicallypinned to the frame at the leading edge of the respective machine toallow a pivoting movement about a vertical axis through the centerlineof the machine to allow for some misalignment of a truck backing intothe rolls. The rolls consequently provide a simple apparatus forrepetitively establishing and maintaining minimum spacing between therear of the trucks and the front edge of the feed hopper. In case of anasphalt paving machine, as the paving machine advances, the truck ispushed forward and advances in an apparently fixed relationship with thepaving machine, hence at a constant spacing with respect to its feedhopper. The material supplied by each truck allows the paver to advancebut a short distance in its operation and frequent truck exchanges atthe front of such feed hopper become a necessary operation to sustain acontinuous paving process.

While state of the art push rolls allow the supply trucks to quicklyposition themselves with respect to the feed hopper to unload thematerial, the sudden contact by the tires of the loaded trucks appearsto be the cause of a possible problem. The problem may be more severewhen the drivers of the supply trucks are attempting to deliver the loadof material in the shortest possible time. It appears that pavingdefects may be caused by the sudden impacts between the rear wheels of afully loaded material supply truck and the push rolls. Considered to bedefects are abrupt changes in the density of the material being paved,such as voids or bumps. They are believed to be caused when the screedof a paver is being jarred. Also, a sudden change in the forward speedof the paver may be the cause of such imperfections or paving defect.

Another inherent deficiency with state of the art push rolls is aninconsistency in positioning the rear of truck beds with respect to feedhoppers. Such inconsistency is the result of differences in trailinglengths of truck beds, differences in the lengths by which truck bedsextend rearward beyond the rear set of tires of such trucks. Whenrepeatability of the dumping position between trucks is desirable, suchinconsistencies present a problem.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a mechanism for eliminatingpaving defects which may be caused by sudden jolts transmitted by supplytrucks backing against positioning devices such as truck rolls inadvance of paving machines.

It is another object of the invention to provide a positioning mechanismfor allowing a truck bed of a material dump truck to become positionedquickly with respect to a material feed hopper in a paving operationwhile minimizing the risk of a jolt by the truck generating animperfection in strip of material being paved.

It is yet another object of the invention to dampen impact forces causedby engagement of truck push rolls at a leading edge of material intakehopper with truck tires of a material supply truck and transmitted tomaterial processing equipment and a work product.

It is, hence, another object of the invention to facilitate thechange-over rate of supply trucks and increase the delivery rate ofmaterials to such road finishing machines as pavers to reduceoccurrences of slow downs or stoppages in paving operations.

It is a further object of the invention to locate the rear of dump bedsof material supply trucks at a most desirable position with respect to afeed hopper while allowing the rear tires of such trucks to be disposedat an indeterminate distance from a front edge of the feed hopper.

It is still another object of the invention to mount push rolls to aleading edge of apparatus adapted to become positioned with respect tothe rear of a truck bed.

According to the invention, vehicle positioning apparatus includes adevice for engaging the rear portions of tires in rolling contact. Thetire engaging device is mounted to a support frame with an interposedmechanism providing resiliently yielding engagement by the tire engagingdevice. The support frame is positioned with respect to the front of amaterial transfer station for transferring material between the vehicleand the material transfer station.

According to a particular aspect of the invention, the positioningapparatus is mounted to the front of a material transfer station whichis a feed hopper of apparatus moving in the direction of the vehicle,and the vehicle is a material supply truck having a tiltable bed.

According to another aspect of the invention, the positioning apparatushas the capability of having the tire engaging device extended orretracted with respect to the front edge of the feed hopper to correctfor variations in a distance between the rear of the tires of the supplytruck and the rear edge of the bed of the truck.

According to another aspect of the invention, a retraction of the tireengaging device is accomplished while the device is engaged with thetires of the supply truck and the supply truck is being positioned withrespect to the feed hopper

A particular method of positioning a bed of a material supply truck withrespect to a material receiving apparatus includes resilientlycontrolling the position of push rolls which engage the truck toalleviate sudden relative motion changes between the truck and thematerial receiving machine.

Advantages resulting from the invention are a more consistent andquicker positioning of material delivery vehicles with respect to amaterial transfer point, such as, for example, a feed hopper ofapparatus related to paving roadways.

BRIEF DESCRIPTION OF THE DRAWINGS

The Detailed Description of the Invention including a detaileddescription of a preferred embodiment thereof will be best understoodwhen read in reference to the accompanying drawings wherein:

FIG. 1 is a simplified partial plan view of the space between a materialsupply truck and a material receiving machine, showing a push rollassembly as an embodiment of the current invention;

FIG. 2 is a side view of the push roll assembly in FIG. 1, as viewedfrom the direction "2--2" indicated in FIG. 1;

FIG. 3 is a simplified partial plan view of another, preferredembodiment of a push roll assembly further including a mechanism forextending and retracting the position of the push rolls;

FIG. 4 is a simplified partial side view of the push roll assembly shownin FIG. 3, as viewed in the direction "4--4";

FIG. 5 is a schematic diagram of a hydraulic system preferred for theoperation of the embodiment of the invention described with respect toFIGS. 3 and 4; and

FIG. 6 is a graphical comparison of contact forces which may beencountered during the positioning of a material supply truck againstthe push rolls of a material receiving machine.

DETAILED DESCRIPTION OF THE INVENTION

In reference to FIGS. 1 and 2, there is shown a push roll assemblydesignated generally by the numeral 10. The push roll assemblyalleviates certain problems which tend to occur with prior art push rollassemblies. A main transverse support beam 11 of the push roll assembly10 supports left and right push rolls 12 and 13, respectively, inspaced, yet axially substantially aligned relationship on opposite endsof the beam. The designation as left and right push rolls is inreference to an arrow 14. The arrow 14 indicates a preferred directionof travel of apparatus 15 (a front edge of which is shown in FIG. 1, forexample). The apparatus 15 may be a paving machine or other apparatusadapted to receive material from a material supply truck (only tires 16of which are shown) while both the material receiving apparatus 15 andthe truck are advancing at substantially the same speed.

Each of the push rolls 12 and 13 are rotatably supported between spacedsupport ears 17 of a respective pivot bracket 18. The pivot bracket 18pivots about a pivot axis 19 of a pivot shaft 20, the shafts 20 beingsupported at their ends in support plates 22 which are attached to thetransverse main beam 11. As mounted in the pivot brackets 18 withrespect to the axis 19 of the pivot shaft 20, the push rolls 12 and 13are disposed to revolve about the pivot shaft 20. A thrust tongue 24extends at a midpoint of its length radially outward from the respectivepivot bracket 18. An outer contact pad 26 of the thrust tongue 24 restsin contact with outer ends 27 of a leaf spring 28. The leaf spring 28 ismounted to the center of an upright flange 29 of the main transversebeam 11 and extends along its length symmetrically outward to both sidesof the center and into engagement with the respective tongues 24 of bothright and left push roll pivot brackets 18.

Torsion springs 32 acting, for example, between a selected support ear17 on each of the pivot brackets 18 and an adjacent one of the supportplates 22 of the beam 11 may be employed to bias the pivot brackets 18toward the leaf spring 28. With such a biasing arrangement, the outercontact pads 26 are urged into contact with the respectively adjacentouter ends 27 of the leaf spring 28, to remain in contact therewith eventhough no truck tires, such as the tires 16 are in engagement with thepush rolls 12 and 13. It should be understood that such a biasingarrangement is solely for the purpose of preventing the push rolls 12and 13 from pivoting downward under their own weight in the absence of apushing force which may be exerted by contacting truck wheels 12 and 13.

The main transverse beam 11 is mounted in a centered position, asindicated by a centerline 34, on a forward extending tongue 35 of thematerial receiving apparatus 15. A preferably pivotable mount, such asby a vertical pivot bolt 36, enables the main beam 11 to pivot about avertical axis 37 to either side from a normal transverse position, asindicated by the arrow 38. Such pivotal mounting of push rolls is knownin the art as an "oscillating" push roll mount. The description relatingto the oscillating mounting of the main beam is for illustrativepurposes only. The pivotal movement of the beam 11 permits the beam toadjust to a minor angular misalignment between the direction of travelof the truck and the material receiving apparatus 15. Such pivotalcapability extends through only a small angle to evenly distributeforces between the left and right push rolls 12 and 13 and no wayaffects the resiliently yielding action of the push rolls 12 and 13 asafforded by the embodiment described herein including the spring 28.

As shown particularly by the alternate position of the exemplary pushroll 13 in FIG. 2, an initial or normal rest position of the push rolls12 and 13 is in a forward pivoted position. As a truck backs with itstires 16 against the push rolls 12 and 13, at first an initial minormisalignment between the longitudinal axes of the truck and the materialreceiving apparatus will be corrected by a transverse pivotal movementor oscillation of the beam 11 about the axis 37. Next, the energyexisting between the material receiving apparatus 15 and the truck willbe transferred and become stored in the spring 28. The pivot bracket 18will undergo pivotal movement as the push rolls 12 and 13 becomedisplaced by the truck tires 16 toward the material receiving apparatus,as indicated by the arrow 41. The pivotal movement of the pivot bracket18 pivots the thrust tongue 24 from its normal rest position indicatedat 43 to a normally engaged position shown in FIG. 2. In reference toFIG. 2, in the course of pivoting from the rest position at 43 to theengaged position, the energy being stored in the spring 28 exerts agradually increasing force through the push rolls 12 and 13 to the trucktires 16 in the direction indicated by an arrow 44. Such force by thepush rolls 12 and 13 accelerates the truck in the direction of the arrow44. As a result, the truck accelerates relative to the speed of thematerial receiving apparatus, such that the impact energy becomesdistributed over an increased time period with a sudden impact peak ofenergy being eliminated by the push roll assembly 10.

It should be understood from the description of the function of thespring 28, that various changes and modifications are possible withoutdeparting from the spirit and scope of this invention. For example, inreference to FIG. 2, it would be possible to rotate the location of thespring 28 and, of course, the location of the corresponding thrusttongue 24 counterclockwise through, for example, a right angle to enablethe spring to act in a substantially vertical direction. Also, spacepermitting, it is possible to change from one type of spring to another,thus replacing the leaf spring 28 with two, a right and a left coilspring which would be disposed to act against the respective thrusttongues 24. The amount of displacement of the spring and its selectedspring constant are indicative of the amount of energy stored. Thespring constant may be selected as a linear constant or a non-linearconstant. Such a non-linearity would allow for normally smooth transferand storage of energy with an accompanying smooth change of relativemotion between the supply truck and the receiving apparatus 15, whileprotecting the operation of the receiving apparatus 15 againstaccidental jolts due to excessive differences in the motion of the two.Another change within the scope of the invention is contemplated inwhich the mechanical spring is replaced with a "hydraulic" spring. Ithas been found that employing a hydraulic system entails furtheradvantages as will become apparent in reference to FIGS. 3 and 4.

Referring now to FIGS. 3 and 4, a preferred embodiment of the inventionimplements certain features described with respect to FIGS. 1 and 2 witha "hydraulic" resilient arrangement. A push roll assembly 50 includes amain transverse support beam 51 to which the left and right push rolls12 and 13 are rotatably mounted. In contrast to the embodiment depictedin FIGS. 1 and 2, the push rolls 12 and 13 are supported in bearings 53of bearing brackets 54 which are solidly attached to the transverse beam51. The transverse beam 51 is pivotably mounted at its center, such asby means of a vertically disposed pivot shaft 55 extending through afork 56 at a leading edge of a sliding guide tongue 58. The guide tongue58 is slidably supported for free sliding movement substantially along acenterline 59 of the material receiving apparatus 15. FIGS. 3 and 4 showportions of a support structure of such apparatus 15. The guide tongue58 is preferably constructed of structural tubing of rectangular crosssection. In support of its uni-directional sliding movement, the guidetongue is restrained in its movement by a guide tube 62 of complementarycross section to eliminate lateral or pivotal movement about thelongitudinal extent of the guide tongue 58. The guide tube 62 ispreferably welded into or otherwise solidly attached to a support frame63 of the apparatus 15, such as between transverse front beam 64 and anintermediate transverse support beam 65. The transverse beam isconsequently free to pivot in a substantially horizontal plane about thepivot shaft 55, and will undergo translational movement or displacementin the longitudinal direction of the material receiving apparatus 15 assupported by the sliding tongue 58.

Such translational movement is controlled by left and right hydrauliccylinders 68 and 69, respectively. Left and right sides, as well asfront and rear are designated again with respect to a contemplatedforward movement of the apparatus 15 in the direction of the arrow 14.The cylinders 68 and 69 are typical dual action hydraulic cylinders,having a preferred 2.5 inch piston diameter. In the preferredembodiment, the cylinders 68 and 69 are mounted on their respectivesides of the apparatus 15 to the intermediate support beam 65 and extendforward through apertures 71 in the front support beam 64. Activators,such as the pistons within the cylinders 68 and 69, and their outwardextending rods are collectively referred to herein as pistons. Pistons72 and 73 are mounted to respective mounting lugs 74 and 75 extendingfrom a rear face 76 of the tranverse beam 51. A transversely spacedposition of the lugs 74 and 75 outward from the centerline 59 of theapparatus 15 is preferred to position the lugs substantially in linewith the thrust exerted by tires 16 of a truck backing against therespective push rolls 12 and 13. Respective hydraulic couplings 77 and78 couple the cylinders 68 and 69 into a hydraulic control circuit 80,which is best described in reference to the schematic diagram of thehydraulic circuit in FIG. 5.

Referring to the schematic diagram of the hydraulic circuit 80 in FIG.5, the cylinders 68 and 69 are coupled in parallel. A hydraulic line 81is coupled between the respective extension couplings 77 of bothcylinders 68 and 69, while a line 82 interconnects the retractioncouplings 78 of the cylinders. Forward or retraction chambers 83 andrear extension chambers 84 of the cylinders 68 and 69 are pairwiseinterconnected and constitute a constant displacement volume.Consequently, for slight angular displacements of the transverse beam 51about the pivot shaft 55 (see also FIG. 3), the left piston 68 extendsor retracts by substantially the same amount as the right piston 69retracts or extends. While combined volume of the two retractionchambers and the combined volume of the two extension chambers willremain constant, a complementary movement of the two pistons in oppositedirections is supported. Thus, without further hydraulic activation, thepreviously described pivotal movement of the transverse beam 51 issupported by the interconnecting lines 81 and 82.

On the other hand, simultaneous retraction or extension of both pistons72 and 73 cannot take place in the absence of other hydraulic controlflow. An initially established position of the pistons 72 and 73 withrespect to their cylinders 68 and 69 results in a fixed, correspondingextension distance "E" of the push roll assembly 50 from a leading edge87 of the front support beam 64 (see FIG. 4). To support resilientlyyielding motion of the push rolls 12 and 13 in accordance with theinvention, the interconnecting line 81 is coupled through an extensionsupply line 89 to a typically gas charged accumulator 90. The amount ofcharge on the accumulator 90 is of course one of choice, though a chargeto a pre-load of 200 pounds per square inch ("psi") has been foundacceptable for typical operations as herein described. The preload to acertain pressure may also need to be evaluated or tested with respect tothe particular diameter of the cylinders 68 and 69 selected. Suchpre-load then becomes the equivalent of a bias force on a mechanicalspring. A compression occurring simultaneously in both extensionchambers 84 with a force resulting 31 in a pressure greater than 200 psicauses the accumulator to store hydraulic fluid against an increasingpressure. Such simultaneous compressive force exerted against thepistons 72 and 73 consequently results in the pistons 72 and 73retracting in the respective cylinders 68 and 69 against thecounterforce exerted by the accumulator 90. The retractile movement ofthe pistons 72 and 73 results in a vacuum being generated in theretraction chambers 83, such vacuum drawing hydraulic fluid through acheck valve 91 and through a retraction supply line 92 from a reservoir93 of the circuit or system 80. The check valve 91 is in parallel with aneedle valve 94 having a variable orifice, the combination of the checkvalve 91 and the needle valve 94 constituting a flow control valve. Inthe operation of the system as described, the additional hydraulic fluiddrawn into the retraction chamber, as a result of the sudden applicationof a compressive force against the pistons 72 and 73, is then slowlybled off through the needle valve 94 to cause the pistons to return totheir originally established positions. The accumulator 90 incombination with the check valve 91 and the needle valve 94 function asa damped spring mechanism to allow the otherwise rigid assembly of thepush rolls 12 and 13 as described with respect to FIGS. 3 and 4, toabsorb and distribute a sudden impact. The accumulator functions in thatrespect in substantially the same manner as that of the embodimentdescribed with respect to FIGS. 1 and 2. One functional distinctionresides in an addition of the damped return of the push rolls 12 and 13in the hydraulic implementation including the needle valve 94. Ofcourse, a wide open setting of the needle valve would reduce the dampingaction. Another physically apparent distinction between the embodimentdescribed with respect to FIGS. 1 and 2, and the hydraulicimplementation of a preferred embodiment described with respect to FIGS.3 and 4 lies in between which elements the resilient absorption takesplace. In the mechanical embodiment, the force of a sudden movement isabsorbed between the push rolls 12 and 13 and the beam 11. In thehydraulic embodiment the force of such sudden movement is absorbedbetween the corresponding beam 51 and the frame of the load receivingapparatus 15 as seen in reference to FIG. 3.

Again in reference to FIG. 5, the hydraulic embodiment described arefurther enhanced by a four-position control valve 95. The extensionsupply line 89 and the retraction supply line 92 are coupled to anoutput side of the control valve 95. The input side of the control valve95 is coupled to a typical hydraulic power supply, including a pump 100coupled to draw hydraulic fluid from the reservoir 93 and to apply itthrough an output line 101 to the pressure input side of the controlvalve 95. In a neutral position 102, shown in the schematic diagram asthe normally engaged or rest position of the control valve 95, the valve95 isolates the power circuit of the pump 100 from the supply lines 89and 92 and returns the hydraulic fluid from the pump 100 through areturn line 103 back to the reservoir 93. A pressure relief valve 104 isactivated by a preset pressure in the output line 101 to route thehydraulic discharge from the pump 100 to the reservoir 93 when thepressure in the output line 101 from the pump 100 exceeds the presetpressure. In the preferred embodiment, the pressure relief valve 104 maybe set, for example, to 1000 psi or another value in accordance withgood engineering practices. It should be realized that in the neutralposition 102, the hydraulic circuit on the output side of the controlvalve 95 functions as the hydraulic force distribution mechanismdescribed above.

The control valve 95 is spring biased from positions adjacent theneutral position 102 toward the neutral position. A push roll extensionposition 105 may be positively activated, such as by manuallyactivation, to couple the pump output line 101 to the extension supplyline 81 and the retraction supply line 82 through the return line 103 tothe reservoir 93. When the extension position 105 becomes positivelyactivated, hydraulic fluid is pumped through the output line 101 and theextension supply line 89 to enter the extension chambers 84 and push thepistons 72 and 73 into an extended position, driving excess fluid fromthe retraction chambers 83 to the reservoir 93. As soon as the controlvalve 95 is released, the valve 95 returns to its neutral position 102to arrest the motion of the pistons in their currently assumedpositions.

A second valve position adjacent the neutral position 102 is a push rollretraction position 106. When the retraction position 106 becomespositively activated, the retraction position of the valve 95 couplesthe output line 101 of the pump to the retraction supply line 92 and theextension supply line 89 to the return line 103. Thus, during anyactivation period of the retraction position, hydraulic fluid is forcedinto the retraction chambers 83, its pressure bearing against thepistons 72 and 73, forcing the pistons to retract and forcing excesshydraulic fluid from the chambers 84 to return to the reservoir 93. Aswith the activation of the push roll extension position 105, a releaseof the push roll retraction position 106 causes the valve 95 to returnto its neutral position 102. In the neutral position 102, the valvearrests further displacement of the pistons to locate the pistons intheir then current positions.

A fourth position 107 of the control valve 95 is what is referred to asa "float position". The float position couples the extension supply line89 and the retraction supply line 92 to each other and couples thesupply lines and the output line 101 to the return line 103. Such aninterconnection of the referred to lines substantially equalizeshydraulic pressures in the extension chambers 84 with respect to thosein the retraction chambers 83. Consequently, the pistons 72 and 73 willremain in a predetermined position as established by prior operation ofthe control valve 95 at one or more of the previously describedpositions. The float position 107 further is a detented position of thevalve 95. Thus, when an operator places the control valve into thedetented position 107, the valve 95 will remain in that position againstthe urging of the spring bias toward the neutral position.

A difference in operation between the float position 10 and the neutralposition 102 will become apparent when a truck backs its tires 16against the push rolls 12 and 13. This distinction will betterunderstood in reference to FIGS. 4 and 5. In an ideal mode of operation,an operator may have previously activated the push roll extensionposition 105. The pistons 72 and 73 are consequently fully extended. Asa truck backs toward the material receiving apparatus 15 and intocontact with the push rolls 12 and 13, the pistons 72 and 73 aresuddenly urged toward the rear of the material receiving apparatus 15. Acheck valve 108 in the supply line 89 prevents the hydraulic fluid fromescaping against the applied force from the extension chambers 84 of thecylinders 68 and 69. The spring action of the accumulator 90 comes intoplay to store and absorb the force resulting from the sudden impactagainst the push rolls 12 and 13. At the same time a needle valve 109which parallels the check valve 108 allows hydraulic fluid to bleed fromthe part of the circuit of the extension chambers 84, whereby a slowretraction of the pistons 72 and 73, and hence of the push rolls 12 and13 takes place. A gauge shown in the diagram of FIG. 5 without numbermay be used as an option if an adjustment of the needle valve 109 ispreferred to be made by observing pressures sustained during theengagement of the needle valve. Thus depending of the adjustment settingof the needle valve 109, a truck, having engaged its tires 16 as shownin the alternate position of the push roll 12 in FIG. 4, may be allowedto further decrease its spacing to the material receiving apparatus 15,until, for example, a lower rear end of a truck, represented by the rearend of its bed 110 is located in a desirable position inboard of aleading or front edge 111 of, and above, a material receiving hopper112. Depending on the amount of overhang of the truck bed 110 beyond therear of the tires 16 a dimension "E" shown in FIG. 4 may differ betweenvarious trucks. In each instance, however, the same relationship betweenthe bed 110 and the hopper 112 may be achieved by allowing the truck toapproach the hopper 112 until the desired relationship is achieved.

When the truck bed 110 is desirably located with respect to the frontedge 111 of the hopper 112, an operator simply releases the detentedposition and the control valve is urged into the neutral position 102.It should be understood, that the operation of the control valve may bemanual or the valve may be automated in accordance with currentpractices. For example, a light beam interrupt mechanism (not shown) orother proximity sensor may be used to determine the correct position ofthe truck bed with respect to the front edge 111 of the hopper 112. Acontrol signal then automatically activates the valve in the describedmanner. On its way to the neutral position the valve briefly engages theextension position 105. Such an engagement is merely transitory, and anyeffect may be an increase of the pressure on the extension side of thecylinder activation circuit. Inasmuch as an effective closing of theneedle valve 109 in any case will result in an increase of hydraulicpressure and an increased absorption of such pressure by theaccumulator, the brief engagement of the position 105 supports thedesired effect of immediately fixing the position of the truck withrespect to the material receiving apparatus 15.

The diagrammatic chart in FIG. 6 gives a comparison of forces describedherein with respect to the resilient absorption of sudden motion andtheir resulting forces. The diagram in FIG. 6 shows three approximationsof forces encountered with respect to time as they are believed to bereceived by the load receiving apparatus as the tires 16 of a truckengage respective push rolls. Graph "A" represents what is believed tobe a typical force distribution when a truck backs against prior artpush rolls. As all slack is taken up, there is a sudden impulsetransmitted between the truck and the load receiving apparatus 15. Thetires 16 are substantially solid because of the extreme load of materialcarried by such a truck. Consequently any difference in motion remainingbetween the truck and the load receiving apparatus is substantiallyopposed and translated into an impulse type force. It is believed thatextreme values of such impacting force may cause sudden movement of theload receiving apparatus 15. Such movement is believed to be sufficientto cause, for example, defects in new pavements when the force istransmitted to a paver.

Graph "B" represents a damped force distribution as a result of theembodiment including the spring 28, as described with respect to FIGS. 1and 2. Graph "B" further applies to the hydraulic equivalent of theaccumulator as described with respect to the preferred embodiment inreference to FIGS. 3, 4 and 5. In both instances, a force level FP isindicated to represent the force required by the load receivingapparatus 15 to push the truck forward by means of the push rolls afterinitial contact transients have become balanced

On the time scale, to represents the rest state of the load receivingapparatus. The base time line may be at an initial bias force of thespring 28 or the accumulator 90 as described herein. As the forcesincrease at t1, relative deceleration between the truck and the loadreceiving apparatus take place. Without resilient force accumulation orenergy storage, at t2 a peak impact force is reached. The spring oraccumulator 28 or 90 delay the rise of the impact force, as indicated byt4, while t5 represents an ultimate time at which transient forces havebecome balanced.

Graph "C" represents a variation of graph "B" as a result of theoperation of the needle valve 109. Bleeding hydraulic fluid from theextension chambers 84 is believed to suppress the force build up in theaccumulator 90, thereby slowing the change of relative motion betweenthe truck and the load receiving apparatus 15. Consequently the graph"C" shows an initially lower resulting force. However, when the valve 95is moved to the neutral position 102, the effect of the accumulator isinvoked. Inasmuch as the truck has during this initial time alreadyadjusted its relative speed substantially to that of the load receivingapparatus, the energy transfer to the accumulator appears less.

Various changes and modifications in the structure of the describedembodiment are possible without departing from the spirit and scope ofthe invention as defined by the terms of the claims appended hereto andreasonable equivalents thereof.

What is claimed is:
 1. A device for cushioning a contact between atravelling paving machine equipped with a hopper for receiving pavingmaterial and a truck carrying paving material to be deposited in thehopper, the truck being equipped with a tire, the device comprising:A.contacting means, connected to the paving machine, for contacting thetruck and for applying to the truck a force tending to move the truck ina direction in which the paving machine is travelling, the contactingmeans movable between an extended position for initially containing thetruck and a retracted position; B. cushioning means, connected to thecontacting means and to the paving machine, for cushioning the forceapplied to the truck and the force applied to the paving machine by thecontact between the contacting means and the truck; C. latch means,connected to one of the paving machine and the contacting means, forlatching the contacting means in the retracted position, the latch meanshaving a latched state and an unlatched state; and D. restoring means,connected to the contacting means and to the paving machine, forrestoring the contacting means to the extended position.
 2. The deviceof claim 1 wherein the latch means comprises:A. a hydraulic cylinderconnected to one of the paving machine and the contacting means; B. apiston movable within the hydraulic cylinder, the piston having a freeend, the free end of the piston connected to the other of the pavingmachine and the contacting means; C. restraining means for restrainingthe piston from moving within the hydraulic cylinder, the restrainingmeans having an inactive state in which the restraining means permitsthe piston to move within the hydraulic cylinder and an active state inwhich the restraining means restrains the piston from moving within thehydraulic cylinder; and D. means for placing the restraining means inthe active state in response to the movement of the contacting means tothe retracted position.
 3. The device of claim 1, further comprising:A.a latch release control disposed to be operable by the operator of thepaving machine; and B. latch release control means, connected to thelatch means, for placing the latch means in the unlatched state inresponse to a single from the latch release control.
 4. The device ofclaim 1 wherein the cushioning means comprises a hydraulic cylinder. 5.The device of claim 1 wherein the restoring means comprises a hydrauliccylinder.
 6. The device of claim 1, further comprising:A. a restoringcontrol disposed to be operable by the operator of the paving machine;and B. restoring control means, connected to the restoring means, foractuating the restoring means to restore the contacting means to theextended position in response to a signal from the restoring control. 7.The device of claim 1 wherein the contacting means is adapted to contacta tire of the truck.
 8. A device for cushioning a contact between atravelling paving machine equipped with a hopper for receiving pavingmaterial and a truck carrying paving material to be deposited in thehopper, the truck being equipped with a tire, the device comprising:A.contacting means, connected to the paving machine, for contacting thetruck and for applying to the truck a force tending to move the truck ina direction in which the paving machine is travelling, the contactingmeans movable between an extended position for initially containing thetruck and a retracted position; B. cushioning means, connected to thecontacting means and to the paving machine, for cushioning the forceapplied to the truck and the force transmitted to the paving machine bythe contact between the contacting means and the truck; C. forcemodulating means, connected to the cushioning means and to the pavingmachine, for modulating the force applied to the truck and the forceapplied to the paving machine by the contact between the contactingmeans and the truck; and D. restoring means, connected to the contactingmeans and to the paving machine, for restoring the contacting means tothe extended position.
 9. The device of claim 8 wherein the forcemodulating means comprises:A. a hydraulic cylinder; B. a piston movablewithin the hydraulic cylinder, the piston defining a front chamber and arear chamber within the hydraulic cylinder, the front chamber containinghydraulic fluid at a first pressure, and the rear chamber containinghydraulic fluid at a second pressure; and C. pressure modulating meansfor modulating the difference between the first pressure and the secondpressure.
 10. The device of claim 8, further comprising:A. a forcemodulating control disposed to be operable by the operator of the pavingmachine; and B. force modulating control means, connected to the forcemodulating means, for controlling the extent to which the forcemodulating means modulated the force applied to the truck and the forceapplied to the paving machine by the contact between the contactingmeans and the truck, the force modulating control means responsive to asignal from the force modulating control.
 11. The device of claim 8,further comprising latch means, connected to one of the paving machineand the contacting means, for latching the contacting means in theretracted position, the latch means having a latched state and anunlatched state.
 12. The device of claim 9, further comprising:A. apressure modulating control disposed to be operable by the operator ofthe paving machine; and B. pressure modulating control means, connectedto the pressure modulating means, for controlling the extent to whichthe pressure modulating means modulates the difference between the firstpressure and the second pressure, the pressure modulating control meansresponsive to a signal from the pressure modulating control.
 13. Thedevice of claim 11 wherein the latch means comprises:A. a hydrauliccylinder connected to one of the paving machine and the contactingmeans; B. a piston movable within the hydraulic cylinder, the pistonhaving a free end, the free end of the piston connected to the other ofthe paving machine and the contacting means; C. restraining means forrestraining the piston from moving within the hydraulic cylinder, therestraining means having an inactive state in which the restrainingmeans permits the piston to move within the hydraulic cylinder and anactive state in which the restraining means restrains the piston frommoving within the hydraulic cylinder; and D. means for placing therestraining means in the active state in response to the movement of thecontacting means to the retracted position.
 14. The device of claim 11,further comprising:A. a latch release control disposed to be operable bythe operator of the paving machine; and B. latch release control means,connected to the latch means, for placing the latch means in theunlatched state in response to a single from the latch release control.15. The device of claim 13, further comprising:A. a restraint controldisposed to be operable by the operator of the paving machine; and B.restraint control means, connected to the restraining means, for placingthe restraining mean in the inactive state in response to a signal fromthe restraint control.
 16. The device of claim 8 wherein the cushioningmeans comprises a hydraulic cylinder.
 17. The device of claim 8 whereinthe restoring means comprises a hydraulic cylinder.
 18. The device ofclaim 8, further comprising:A. a restoring control disposed to beoperable by the operator of the paving machine; and B. restoring controlmeans, connected to the restoring means, for actuating the restoringmeans to restore the contacting means to the extended position inresponse to a signal from the restoring control.
 19. The device of claim8 wherein the contacting means is adapted to contact a tire of thetruck.